Non-Return Valve - Brake Vacuum - the one-way valve in the vacuum line between the engine and the brake servo, part of the master cylinder and servo assembly. The valve lets air be drawn out of the servo chamber while the engine is running and seals against reverse flow the moment vacuum falls away, so a vacuum reserve is held in the servo. That reserve is what provides assisted braking for a number of pedal applications after the engine stops. A valve that leaks or sticks open shows up as a hard brake pedal, inconsistent servo assistance, or an audible hiss from the bulkhead area. The valve is directional and must be fitted the correct way round. Renew the sealing grommet in the servo body at the same time if it has hardened or perished, since a poor seal there produces the same symptoms as a failed valve.
